I have turned off the mic when I asked if you can hear the hum directly from the transformer. It turned out the ballroom was too noisy for the mic to pick up hum directly so we had to keep the guitar amp on to amplifier the growl and hum.

Next time I will find a way to show you the vibration of the transformer makes with DC is fed into it from the powerline. It will really tick you off when you spend all the money of external vibration control and the most of the vibration are coming from the internal transformer caused by DC offset from the powerline.